Jan/Feb is notorious timing for attraction refurbishments (usually water related rides, i.e Splash Mtn) and others as well. Weather is better, it isn't hurricane season, swimming may be iffy.
In September you've got Mickey's Not So Scary Halloween Party, Food and Wine, the heat, weather, etc.
I personally have not been in the Jan/Feb time frame but we are planning to do so when we take all our kids in early 2024. I would research both time frames and see what feels right for your crew. There are many good resources out there.